The continuation of this prospective international cohort study of patients after acute unilateral anterior cruciate ligament will help influence the care of the 200,000 or more Americans who rupture their ACL's each year by answering important clinical questions regarding the role and impact of dynamic knee stability on patient outcomes. The inclusion of the international sample is allowing us to test the conventional wisdom that drives surgical decision-making in the treatment of ACL rupture in the United States. In addition, the further elucidation of how those with different early compensation strategies for the injury are affected by neuromuscular training and reconstructive surgery can allow us to derive and test meaningful prediction rules for the management of these individuals. Our nearly ten year collaboration with Oslo University Hospital in Norway, where the practice pattern requires a substantial period of rehabilitation before they undergo reconstructive surgery, provided the platform for this unique cohort. To answer these questions, our international, multidisciplinary team has performed clinical and functional evaluation in persons classified by our screening examination before and after unilateral ACLR. The important results we obtained from the past five years demonstrate that there is a differential response to ACL injury that can be affected by rehabilitation, but also demonstrated that as rehabilitation continues before surgery, stability strategies change. Categories are fluid, non-copers can become more stable, potential copers can become unstable and while surgery introduces passive stability, successful outcome is not inevitable. Intent to return to previous level of actiity is not a predictor of actual return. Prediction of medium term (1 year) success from modifiable impairments and physical performance measures is robust and prediction is better after a period of rehabilitation than acutely after injury. We have an extremely rich existing base of data (cohorts of 130-150 active individuals at each site were enrolled within 3 months of injury and followed prospectively) with more than a 90% on site follow-up at 1 year and a tremendous opportunity to follow this well-characterized sample. Longitudinal data have been used to assess response to rehabilitation and surgery, return to activity and reinjury in the medium term (up to one year). Continued follow-up of this cohort will allow us to focus on many important questions with a continued goal of examining similarities and differences between the surgically treated and the non- surgically treated group and the US and Norwegian samples 2 and 5 -7 years after injury. The proposed studies will extend our previous findings and will provide important evidence that will inform the treatment of individuals with ACL rupture across the spectrum of compensation strategies and treatment options. . PUBLIC HEALTH RELEVANCE: This international cohort proposed studies will begin to elucidate the particular clinical markers that contribute to the success or failure following ACL rupture and reconstruction and/or return to full activity and provide clinicians with practical, useful and evidence-based treatment options that may improve function after ACL injury or reconstruction.